The present invention is directed generally to microwaves, and, more particularly, to materials employed for absorbing microwaves.
Microwave absorbing material is valuable in a variety of applications. The most notable applications include anechoic chamber walls and stealthy aircraft and missile skins.
The typical microwave absorbing materials used in anechoic chambers are ferrites and polystyrene. These materials are expensive and lack the strength to be used in aircraft and missile skins. Further, these materials have fixed ranges of operation and are not tunable.
Minimization of radar reflectivity is of varying importance in different kinds of military missions. Avoidance of detection is often a paramount consideration.
Varieties of approaches have been taken. One such approach discloses altering the construction of the aircraft as well as fabricating the shell of the aircraft from a rigid structural foam, which is filled with a microwave energy absorbing or dissipating material. Carbon or iron or nichrome are listed as possible fillers. See, e.g., U.S. Pat. No. 5,016,015, issued May 14, 1991, entitled xe2x80x9cAircraft Constructionxe2x80x9d.
Another approach discloses chemical tuning to modify the microwave dielectric and/or magnetic properties of a microwave-absorbing material. The microwave-absorbing material comprises blends of polar icosahedral molecular units with a variety of host matrices, or with polymers with units covalently bonded in a pendant manner to the polymer chain. See, e.g., U.S. Pat. No. 5,317,058, issued May 31, 1994, entitled xe2x80x9cMicrowave-Absorbing Materials Containing Polar Icosahedral Units and Methods of Making the Samexe2x80x9d.
Finally, another area of use of microwave absorbable materials is in anechoic chambers. A problem in anechoic chambers is that reflections from the walls may interfere with the scattering results from the object under test.
Thus, there remains a need for a microwave-absorbing material that is relatively lightweight, is structurally sound, and exhibits a high absorption coefficient. Additionally, such material ideally should be tunable in real time.
In accordance with the present invention, a method of absorbing microwave radiation is provided. The method comprises placing a structure in the path of the microwave radiation, the structure comprising an array of metal plates supported over a metal substrate by vertical conducting vias.
Also in accordance with the present invention, a missile having a dome portion is provided that operates in a stealth mode. At least the inside of the dome portion is provided with the above-described structure for absorbing microwave radiation.
Further in accordance with the present invention, an anechoic chamber for use in testing microwave-emitting devices is provided. Such anechoic chambers have walls, a floor, and a ceiling. The walls, the floor, and the ceiling are provided with the above-described structure for absorbing microwave radiation.
The present invention uses surface patterning to enhance the microwave absorption. The cost can be reduced by using surface patterning rather than using more exotic materials. Furthermore, the material can be substantially stronger than was previously available using ferrite-based materials, thus allowing the absorbing material to be more easily integrated into the skins of aircraft and missiles. In addition, the frequency over which the material is highly absorptive can be shifted by changing the height of the structure, thus allowing active control (xe2x80x9ctunable in real timexe2x80x9d). A major immediate use of this material may well be for anechoic chamber walls, where the reduction of multiple reflections from the walls will improve the sensitivity of the measurements by reducing their interference with the scattering results from the object under test. It requires less volume to implement when compared to the conventional passive absorbers.